Close coupled gas atomization of liquid metal is being developed as a process for forming metal powders. The close coupled gas atomization process is performed by a nozzle comprised of a melt guide tube extending axially through a cylindrical gas plenum. The cylindrical gas plenum has an inner chamber in communication with an annular orifice, or an annular array of discrete orifices, so that a gas flow therethrough produces an atomizing gas jet which may be comprised of an array of discrete jets. The gas jet has a conical shape converging below the melt guide tube. A stream of liquid metal passing through the melt guide tube and exiting therefrom is atomized by the conical gas jet converging in the stream.
The flow rate of liquid being poured from a vessel through the atomizing nozzle is an important parameter in the atomization of liquid metals. Improved control of the atomization process can be provided with an accurate substantially instantaneous measurement of the flow rate of the liquid metal poured from the vessel. One problem that can occur during the atomization process is liquid metal freezing in the melt guide tube, herein referred to as freeze-off . A substantially instantaneous measurement of the liquid metal flow rate through the nozzle could provide a warning of impending freeze-off.
However, conventional flow rate sensors such as venturi meters and vibrating tube flow meters are unsuitable for measuring the flow rate of the high temperature liquid metals. An apparatus utilizing an eddy current sensor for measuring the flow rate of liquid metal is under investigation, see the NIST/Industrial Consortium on Intelligent Processing of Rapidly Solidified Metal Powders by Inert Gas Atomization, Semi-Annual Report, Mar. 1 to Aug. 31, 1992, National Institute of Standards and Technology, Gaithersburg, Md.
Several important properties of metal powder, and the products formed from consolidation of the powder, are dependent on the as-atomized particle size. These properties include composition homogeneity, mechanical performance, e.g. strength, and toughness, as well as physical characteristics of the powder itself, e.g., particle shape, porosity, and flow qualities. Most of these properties improve as particle size decreases, however, powder handling becomes more complicated for finer powder because of caking, environmental contamination, pyrophorosity and other affects.
The strong dependence of material properties on particle size translates into an increased demand for atomization process control that provides a predetermined particle size range, and minimizes the production of powder having a particle size above or below the predetermined range. At the same time, a number of variables necessarily change during the atomization process, such as the flow rate of molten metal through the nozzle as the static head pressure of the liquid metal in the supply vessel changes. As a result, a series of adjustments can be required during the atomization process in response to the changing variables.
An aspect of this invention is to provide an apparatus and method for atomizing liquid metal utilizing a substantially instantaneous measurement of the flow rate of the liquid metal.